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POPULAR SCIENCE REVIEW. 
Etna. By prolonging this plane to the East, we see it passes through the 
principal volcanic centres of the Mediterranean, and even those on the borders 
of the Caspian sea. — Vide Comptes Rendus, March 30. 
The Fossil Water-hen of Mauritius. — ^M. Alphonse Milne Edwards, who 
sometime since described this fossil Fulica to the French Academy, says that 
the species (F. Newtonii) was of great size, that it is now extinct, and that 
it existed contemporaneously with the Dodo (?) Dubois, who visited the 
Mascarene Islands in 1669 to 1672, in describing the birds of Bourbon, speaks 
of water-hens as large as domestic fowl, all black, with a white crest on 
the summit of the head.” This description cannot apply to the existing 
F, cristata, which is much smaller than an ordinary fowl. The bones of F. 
Newtonii examined by M. Edwards enable him to state that the extinct bird 
must have been about the size of a common hen, and hence he concludes 
that this species was the one spoken of by Dubois in his writings. — Comptes 
Rendus f tom. LXVI., No. 13. 
The Probahle Date of the Glacial Pei'iod. — Under this title Mr. James 
Croll has contributed to the Philosophical Magazine (May) a paper of much 
interest to the speculative geologist. Mr. Croll has not concluded his re- 
marks in this memoir, but promises to do so in another. Nevertheless, we 
have much pleasure in calling attention to a most important paper on one of 
the grandest problems that can engage the attention of the man of science. 
We may mention, that Mr. Croll looks upon denudation as the key to the 
solution of the question — How old is the earth ? He thinks that accurate 
ideas of denudation can only be formed when enough evidence concerning 
the quantity of sediment carried away by our rivers is accumulated. 
MECHANICAL SCIENCE. ' 
Stresses in Braced Structures.— Mx. W. Airy has communicated to the 
Institute of Civil Engineers a paper on the experimental determination of 
the stresses in complex braced structures, and particularly in bowstring 
girders, where the labour of calculation is considerable. Mr. Airy’s method, 
which is entirely new, and very interesting, consists in constructing a model 
of the structure, in which all the bracing consists of thin steel wires, very 
accurately adjusted as to length, so that when unloaded none of the bracing 
is subjected to stress. The structure is then loaded so as to bring the 
bracing bars into tension, when, on being pulled, each of the bracing bars 
will give a musical note, the pitch of which depends on the stress. To 
ascertain this a free wire, precisely similar in size to the bracing bar, is sus- 
pended in a frame and cut off to the same length as the bracing bar by a 
sliding bridge. The free string supports a small scale pan, and this scale 
pan is loaded with weights, till its note corresponds with that of the bracing 
bar, the stress on which is required. This was determined by ear with the 
greatest accuracy, the effect of | oz. in 80 oz. being clearly perceptible. The 
tension of the bracing bar was thus measured by the weight in the scale 
pan of the free string, and the. process is repeated for. every bracing bar in. 
the structure. The determination of the thrusts is made by a differential 
